Parallel plate type electron beam arrays are known. Presently, such arrays are being provided in the form of microminiature field emitters, which are known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as electron sources in flat panel displays for use in aviation, automobiles, workstations, laptops, head wearable displays, heads up displays, outdoor signage, or practically any application for a screen which conveys information through light emission. Field emitters, as well as other types of electron beam arrays, may also be used in non-display applications such as power supplies, printers, and X-ray sensors.
Referring to FIG. 1, the cross-section of a parallel plate type electron beam emission device 10 is shown. The device includes a bottom plate 100, a spacer structure 200, and a top plate 300. The bottom plate 100 may comprise a substrate 110 and a conductive element 120. The bottom plate 100 may include additional elements in the interior of the device 10 including conductive gates, which are useful for emitting electrons in the direction of the top plate 300. The top plate 300 may comprise a substrate 310 and a conductive element 320. The top and bottom plates may be connected along their respective outer edge regions with the spacer structure 200. The spacer structure 200 may itself comprise an insulator frame or ring 210 bonded to the top and bottom plates with an upper glass frit 220 and a lower glass frit 230, respectively.
In order to achieve a beam of electrons, from the bottom plate 100 to the top plate 300, of a predetermined velocity, the upper conductive element 320 may be maintained at a high positive voltage relative to the source of electrons located on the bottom plate 100. Thus the upper conductive element 320 may also be referred to as an anode. If the device 10 is a display, the anode 320 may be implemented by a thin transparent conductive layer.
In order to operate the device 10, the space between the bottom plate 100 and the top plate 300 should be evacuated. Typically, this space may be of the order of 0.5 to 5 millimeters. To maintain the vacuum between the top and bottom plates, they are sealed to one another along their respective edges by the spacer structure 200. After being sealed, the space between the two plates, 100 and 300, may be evacuated of air or gas and sealed off from the outside atmosphere.
It is imperative to the operation of the device 10 to have as near to a perfect vacuum in the device as possible. The reason being that gas molecules within the device may become ionized as a result of being bombarded by the electrons in the device. If the gas pressure is high enough, there will be a growth in the ionization leading to a gas-discharge (breakdown flashover) between the anode 320 and the elements of the bottom plate 100. In devices in which the potential between the anode 320 and the bottom plate 100 is in the range of thousands of volts, such flashover may be catastrophic to the device 10. The flashover problem is particularly noticeable during the burn-in of new displays. Burn-in is carried out by operating a display at anode voltages well above those that would be experienced by the display during normal operation. It is at this time that displays are particularly susceptible to flashover.
The susceptibility of a display to flashover may be related to the density of gas in the region of the display where the flashover occurs. The density of gas molecules close to the display wall tends to be high on a short time scale. If the product (p)(d) of the local gas pressure (p) in the vicinity of the walls and the distance (d) between the anode and the gate is sufficient for a Paschen breakdown, then a cumulative ionization leading to a gas discharge (flashover) will occur between the anode and the gate. The flashover between the anode and the gate can trigger a flashover between that gate and corresponding emitters. For this reason most flashovers take place close to the sidewalls in a field emission display.
Prior to the present invention, adequate flashover control at high voltages (e.g., .gtoreq.6KV) has been difficult. The primary method of combating flashover has been to reduce the operating potential between the anode 320 and the elements of the bottom plate 100. By decreasing the potential to levels of only a few hundred volts, the occurrence of flashover may be reduced, although it is far from eliminated.
Ise, U.S. Pat. No. 5,448,133 (issued Sep. 5, 1995) for a Flat Panel Field Emission Display Device with a Reflective Layer, touts the advantages of reducing the potential between the anode and cathode in a Field Emitter Display (FED). Ise states that a reduction of the operating voltage of a FED will reduce power consumption, which reduces battery size, and enables portability. Ise states that presently the low end threshold for anode to cathode potential is about 400 volts. Ise reports operation of his FED at as low as 100 volts of cathode to anode potential.
Reduction of the bottom plate to anode potential, however, as suggested by Ise, may reduce FED lifespan. Lifespan may be reduced because the luminous efficiency of the FED phosphors depends on the coulomb charge per unit volume applied to the phosphors over a period of time. The application of charge to the phosphors seems to dislocate activators from their sites in the phosphor host lattice, and thus decreases the activator excitation efficiency (by increasing the vacancy density). A phosphor layer of certain thickness, if operated by high voltage and low current, tends to have low values of coulombs per unit volume due to the increased penetration depth of the charge delivering electrons. On the other hand, if the same layer is operated with low voltage and high current (maintaining the same power) the coulombs per unit area increases because of the increased current, and the coulombs per unit volume increases even more due to the decreased penetration of the electrons (charge concentration at the surface of the layer). Increased coulomb density resulting from low voltage operation is more detrimental to the activators than high voltage operation over a given time span. Consequently the luminous efficiency decreases more rapidly for low voltage FED's. A decrease in light output may also occur in low voltage FED's due to the intervening passive thickness of the phosphor layer between the observer and the active surface layer.
The problems associated with sidewall induced flashovers, discussed above, may also arise in the interior portions of large sized screen FED's when low internal device pressure is maintained. Internal spacers are commonly used in FEDs to prevent the FED screen from bowing inward as a result of the pressure difference between atmosphere outside and the vacuum conditions of the FED interior. While the spacers beneficially keep the screen from bowing or breaking, the spacers also provide a surface linking the gate and anode which can facilitate flashovers. Trace residual gas or gas buildup on these surfaces can support plasma arcs.
Accordingly, there is a need for new methods and apparatus for reducing the occurrence of flashover, without reducing the level of anode voltages. There is also a need for methods and apparatus for reducing the magnitude of damage suffered from the occurrence of flashovers during the initial burn-in and operation of the device. There is a particular need for a device which does not readily support surface flashovers along the interior surfaces and/or internal spacers of the device. The present invention meets this need, and provides other benefits as well.